The hyperlipemia of pregnancy consists of elevations in plasma triglyceride, cholesterol, and phospholipid. The meaning of these changes in terms of plasma lipoproteins is not clearly established. Likewise, plasma lipoprotein alterations in pregnancies complicated by diabetes, obesity, familial hyperlipemia, or other diseases are poorly described. No correlation has been attempted between maternal lipids in these disorders and neonatal birth weight and survival. Basic physiological questions emerge from these clinical problems. Is the lipemia of pregnancy due to an increased rate of lipid entry into the vascular compartment, a decreased rate of removal, an abnormal lipid particle, or some combination of these? To answer these questions, the turnover of intravascular lipids should be studied, but the lipid metabolism of tissues and organs responsible for lipid ingress and egress must also be understood. Lipoproteins are formed in the intestine from dietary fat and in the liver and intestine from endogenous fat mobilized from adipose tissue stores. On leaving the circulation, lipoprotein triglyceride may be stored in adipose tissue or oxidized for energy at other sites including the placenta and the pregnant uterus. When the factors affecting lipid formation and degradation are measured, it should be possible to explain in kinetic terms not only the hyperlipemia but the increased adipose tissue mass of pregnancy as well. While each of these problems merits individual study, their collective solution bears on another important issue. As in the starved pregnancy, the normally feeding mother in the third trimester may turn to lipid fuels for energy to spare glucose and amino acids for the growing fetus. The increased adipose tissue mass accumulated earlier in gestation could meet the increased demand for lipid fuels, and the elevated plasma lipids and free fatty acids could provide the means of delivery. If we can prove that the mother oxidizes more lipid in late gestation from these sources, generous maternal weight gain earlier in pregnancy can be justified on a scientific basis.